Tattooing is not disabling, it is treating the skin as a canvas, and creating a piece of art ( or advertising ) Expression in a semi- permanent form, which you carry with you.
I don't get it either, i'm not interested in the cost, nor the pain, but mostly the cost. However i wouldn't class it with other forms of body modding. They may indeed be more forms of 3

sculpture, but at present they are mostly rather primitive ( at least compared to ink, modern tattoos can be pretty good, and safe )
But body modding which disables, because you feel uncomfortable in your own body is more an expression of self, than an expression of art. If that distinction makes sense.
I'd be much more comfortable with artistic disfiguration. Just because i'm not into it, doesn't mean i think it should be banned. ( and i'd be more likely to want a super body than a disabled one... )